signal theory

Signal Theory Toward a Unfied Theory of Psychedelic Actionor: “Hallucinogens and Recurrent Excitation in Cortical Circuitry”

· Tryptamine hallucinogens are most active at the 5-HT2A receptor subtype.· The 5-HT2A receptor subtype is densest in the recurrent cortical circuits of the

sensory processing pathways.· An increase in recurrent feedback excitation within the sensory processing cortices

would lead to sensory signal gain, obsessive repetition of data, morphic distortionof data, and temporal lag in multi-modal sensory convergence.

· Thus, increased recurrent excitation within cortical circuits is the direct cause ofperceptual distortions, hallucinatory constructs, and expanded states ofconsciousness associated with the psychedelic state.

SUMMARY

RECURRENT EXCITATION IN ANALYTICAL SENSORY PROCESSING

REFERENCESG. Aghajanian, G. Marek; Serotonin, via 5-HT2A receptors, increases ESPCsin layer V pyramid cells of prefrontal cortex by an asynchronous mode ofglutamate release; Brain Research 825 (1999) 161-171.

P. Bressloff et al.; What Geometric Visuals Tell Us about the Visual Cortex;Neural Computation 14 (2002) 473-491.

M. Diamond et al.; The Human Brain Coloring Book; Harper, N.Y. (1985) 5-32.

R.J. Douglas et al.; Recurrent Excitation in Neocortical Circuits; Science, 269(1995) 981-985.

RECURRENT EXCITATION IN VISUAL PROCESSING PATHWAYSThe sensory processing pathways in the cerebral cortex contain many analyticalcircuits that derive processing power from the recurrent parsing of sensory input.Each recurrent circuit has a specific analytical function, and ambiguous oremotionally salient data may be re-routed through the same circuit multiple timesto boost signal fidelity and enhance data resolution. As incoming sense dataresolves through each circuit, output is fed forward as a progressive scan witha delay, or lag, of a few milliseconds for each sucessive circuit iteration.

As recurrent circuits become excited, they act as signal attractors, or “traps”, forany incoming data that requires refined analysis. Data that is being activelyscrutinized must be held in recurrent analysis until the integrative threshold ofthat specific data set has been reached. Once the circuit resolves the data, itdrops the recurrent signal and moves onto the next piece of sensory input, allthe while relying on instantaneous feedback from downstream circuits to confirmdata reception and/or request further analysis on a specific set of data.

When these recurrent circuits become excited in the presence of a hallucinogen,all incoming sensory data is “trapped”, intensified, compulsively scrutinized, andobsessively analyzed. This can result in expanded states of awareness as wellas pathological ideation and/or psychotic distortions of incoming data. Theperceptions which arise from this state may be amazingly perceptive or cartoonishlyabsurd, depending on the dose of hallucinogen taken.

Human perception relies on a complicated network of recurrent circuits acting in unison to createa sensory gestalt of our immediate environment in working memory. The multi-modal convergenceof sight, sound, smell, and touch relies on strong signal coupling between multiple sensory circuits,and the stability of this coupling relies on precise signal output timing from all circuits. If specificcircuits become overly excited or overly inhibited, a divergence in circuit synchrony can createcircuit instability that propagates throughout the entire network as a frame-translation error.

A good example of a simple frame-translation error can be found when analyzing the structure ofphosphenes, the closed-eye geometric patterns seen when the optic nerve is excited. These swirlingmandalas of the mind have been reproduced for thousands of years in spiritual artwork, but wenow know that such patterns may be the direct result of instability in signal coupling between thespatially oriented neural structures in the retina and the visual cortex (see illustration to the left).If this instability is caused by excitation and lag in the feedback circuit connecting the LGN withthe visual cortex, one would expect to see phantom frame data drifting across the visual fieldwhenever the eyes were closed, creating the perception of spinning, luminescent mandalas.

Another example of a simple frame-translation error is a perceptible strobing of visible light, orframe-flicker, caused by divergent excitation between the optic nerve, the LGN, and the visualcortex. A more extreme version of this effect is total visual frame feedback, or flanging, wheresmooth perception is sliced into discrete frames of data which pile up on each other like visualechoes, often with a slight rotational effect. Flanging is the direct result of lag in recurrent signalconvergence along the entire visual processing pathway.

SIGNAL EXCITATION, STABILITY, AND FRAME TRANSLATION ERRORS

Recurrent excitation is just another word for feedback, the process in which a portion of the outputof a specific circuit is redirected back into its own input for further analysis. Feedback excitationin cortical circuits is used to amplify signal, improve signal fidelity, and refine detail resolution.

Recurrent excitation may be thought of in terms of an optical scanner that must make multiplepasses over the same image to acheive high levels of detail resolution. Each successive feedbackpass refines detail resolution and enhances sigal fidelity.

Feedback excitation in cortical circuits is responsible for signal analysis, data discrimination, detailresolution, manipulation of data in working memory, compression of data in long-term memory,fine motor control, as well as many other extremely important cognitive functions.

An interruption of recurrent excitation could lead to disorientation, loss of cognitive focus, loss ofmemory recall, and loss of fine motor control.

An increase in recurrent excitation could lead to amplification of sensory signal intensity, enhanceddetail resolution, enhanced analytical processing, and states of expanded consciousness.

Runaway (unchecked) feedback excitation could lead to perceptual distortions, obsessive ideation,hallucinations, loss of synchrony in multi-modal sensory cohesion, psychosis, catatonia, and (atthe extreme end) epileptic seizure.

F. Forutan et al.; Distribution of 5-HT2A receptors in the human brain;Nuklearmedizin 41 (2002) 197-201.

W. Gao et al.; Presynaptic regulation of recurrent excitation by D1 receptorsin prefrontal circuits; PNAS 98-1 (2001) 295-300.

B. Gutkin, D. Pinto, B. Ermentrout; Mathematical neuroscience: fromneurons to circuits to systems; J. Physiology, Paris 97 (2003) 209-219.

J.M. Hupé et al.; Cortical feedback improves discrimination between figureand background by V1, V2 and V3 neurons; Nature 394 (1998) 784-787.

J. LeDoux; Synaptic Self: How Our Brains Become Who We Are; VikingPress, N.Y. (2002) 175-199.

E. Lumer, G. Edelman, G. Tononi et al.; Neural dynamics in a model of thethalamocortical system. II. The role of neural synchrony tested throughperturbations of spike timing; Cerebral Cortex, Vol 7 (1997) 228-236.

Z. Shao, A. Burkhalter; Role of GABAB Receptor-Mediated Inhibition inReciprocal Interareal Pathways of Rat Visual Cortex; American PhysiologicalSociety (1999) 1014-1024.

Recurrent cortical circuits are subject to both internal feedback(open loop excitation) as well as upstream feedback fromdownstream circuits (closed loop feedback).The total amountof circuit feedback depends upon the intensity of feed-forwardand feedback input stimulating the circuit at any one time.

RECURRENT EXCITATION AND PARANORMAL PERCEPTIONThe primary area where recurrent circuitry is used to amplify incoming visualsignal is in the visual cortex. The layer V pyramid cells of the visual cortex containmore 5-HT2A receptors than any other part of the brain, and all of these receptorsare used to modulate recurrent excitation both within the visual cortex and betweenthe visual cortex and the LGN. Excessive recurrent excitation in this feedbackcircuit is responsible for the intensification of colors and morphic distortion in lineand texture resolution associated with psychedelic states.

From the visual cortex, signal diverges and feeds-forward to the analyticalprocessing centers of the temporal and parietal lobes. The temporal lobe circuitryis engaged to determine “what” is being viewed, while the parietal lobe circuitrydetermines “where” everything is in relation to everything else. Both of thesecircuits rely on recurrent excitation to preserve moment-to-moment contextualdata as well as provide robust analysis of incoming data.

When analyzing the trajectory of a moving object, data must pass back and forthbetween the visual cortex and the spatial cortex multiple times in order to updatethe location of the moving object over time. Excessive excitation in this recipricalcircuit causes the trajectory of the moving object to decay from visual memoryat a much slower rate than usual, thus causing moving objects to leave “trails”in the field of vision under the influence of a hallucinogen. The time decay of thesevisual trails is directly proportional to the intensity of feedback within the circuit,which is dependent on the dose and receptor affinity of the hallucinogen.

While visual trails are easy enough to illustrate, another intriguing phenomenonarises when recurrent circuitry between the visual cortex and the object recognitioncortex becomes excited. In this state, each object that is scrutinized immediatelytakes on deep significance. Everyday objects seem loaded with history andpersonal meaning; simple textures and odd shapes reveal hidden images andburied memories; the mundane suddenly becomes infinitely fascinating. Thisexcercise of finding deep understanding in the simplest of objects is often referredto as “enlightenment” or an “expanded state of consciousness”.

Dilated Pupil

ExpandedPeriphery Data

Retina

VisualInput

Optic Nerve

Lateral GeniculateNucleus (LGN)

Temporal LobeObject Recognition

Occipital LobeVisual Cortex

Parietal LobeSpatial OrientationPrefrontal Cortex (PFC)

Multi-modal sensoryconvergence

PERCEPTUAL DISTORTIONS CAUSED BY RUNAWAY RECURRENT EXCITATION

The human brain has a naturalability to find patterns in otherwiserandom noise, and this ability relieson recurrent analysis of visualstimulus. Psychedelics amplify thisability by increasing excitation inthe object recognition cortex of themedial temporal lobe. In an excitedstate, the brain can discern and“paint” elaborate patterns on anyfield of noisy data, such as TVstatic, concrete (above), and otherrandomly distributed textures.

STOCHASTIC SMOOTHING

RecurrentAnalytical

CircuitOutput

FeedbackFeedback

Input

Recurrent Excitation

MATHEMATICAL MODEL OF A SIMPLIFIED RECURRENT CIRCUIT

RecurrentCircuit

IffFeed Forward Input

Irec Recurrent Excitation

aF Excitatory Conductance

bF Inhibitory Conductance

G Circuit Conductance

F*Feed Forward Output

Recurrent Excitation: Irec = (a - b)FF* Firing Frequency: Iff + (a - b)F* = GF*Total Circuit Gain: 1/(G + b - a)

The illustration to the left shows a simplified model of a recurrent circuit in the visualcortex. The variable Iff represents the amount of feed-forward input being sent to thecircuit, the variable G is the general conductance of the entire circuit, and the variablea represents the excitatory conductance of the circuit. In this model, the excitatoryinfluence of a hallucinogen would be applied to the variable a, which enhances the totalcircuit conductance and, in the absence of proportional inhibition (variable b), amplifiessignal gain across the circuit.

As the excitatory conductance of a rises, there is a chain reaction of responses withinthe recurrent circuit. First, the effective conductance of the circuit rises, causing thelevel of recurrent excitation (Irec) applied to incoming signal (Iff) to rise in direct proportion.The result is both an amplification of signal intensity as well as a slight delay, or lag,in feed-forward signal integration proportional to the gain on that individual circuit.Divergent lag in recurrent cortical circuits leads to loss of synchrony in multi-modalsensory convergence, causing spatio-temporal distortion of incoming sense data.

Due to the slow asynchronous release of glutamate in the presence of a 5-HT2A partialagonist, the value of a would be expected to rise steadily over the duration of an entirepsychedelic trip. As the gain on a specific circuit rises and approaches 1, neurons withinthat circuit will quickly reach maximal discharge in the presence of even the smalleststimulus. If the circuit gain exceeds 1, no external stimulus is needed to promote circuitactivity; recurrent excitation alone is enough to keep the circuit actively feeding ‘phantomechoes’ of sense data forward.

The above panels show bifurcating patterns in spatially oriented cortical (a) and retinal(b) networks. The top panels show patterns arising from distance-dependent connections,the bottom show distance- and orientation-dependent connections (Bressloff, et. al).

One of the most common effectsof psychedelics is visual trails.Afterimages of moving objectsremain stuck in visual memory,creating smooth visual echoeswhich fade over a period of a fewseconds. Recurrent excitation alongthe visual processing pathway trapssensory input from moving objectsin the visual cortex the same waya camera’s “time lapse” shuttercaptures moving lights on opticalfilm over time.

People under the influence ofpsychedelics often sense extremedistortion in perceptions of self andenvironment. A room may appearto “close in” as if being observedthrough a fish-eye lens. Conversely,parts of the body may seem toballoon to extreme proportions.These distortions in perspectiveare due to recurrent signal gain inthe spatial and somatic cortexes,both expanding and contractingperceptions of space.

TRAILS AND AFTERIMAGES PERSPECTIVE DISTORTIONAs excitation is slowly introducedinto the recurrent network, moreand more signal is fed back uponi tse l f w i th ever- increas ingdivergence in signal lag. Theperceptual result is similar tofeedback created when a video-camera is pointed at its own output.Discrete slices of perception beginto pile up on each other with a delayof a few milliseconds for each pass,creating an overlapping frame spiralthat reaches toward infinity.

As the psychedelic trip hits its peak,the amount of signal noisegenerated by internal recurrentexcitation begins to overshadowexternal sensory stimulus. At thispoint, the trip becomes “mostpsychedelic” as any stray thoughtor stimulus emerges holographicallyinto consciousness as a self-referential fractal of infinite meaning.A simple sunflower recedes into anepic symbol of truth that can begrokked over and over, ad infinitum.

OPTIC FRAME FLANGING FRACTALLY RECURSIVE FLANGE

Photo: Stewart Updegrave

EXPANDED CONSCIOUSNESS AND RECURRENT IMPULSE FLOW CONTROLIn shamanic practice, the role ofthe Shaman is to help the patientnavigate the various levels of thepsychedelic journey. Presumably,the Shaman does this by mediatingand fine-tuning the intensity andsynchrony of recurrent feedbackwithin the patient’s sensoryprocessing pathways. This may beaccomplished through precise dosetargeting or other ritual means.

OPTIMAL RATES OF RECURSIONBoth 5-HT and Dopamine (DA)work together to mediate recurrentexcitation. There are pre- and post-synaptic DA receptors that work inunison with 5-HT to focus and fine-tine recurrent excitation. And thoughslow lateral GABA inhibition is allbut overpowered by runawayrecurrent excitation, fast GABApathways can still interrupt andredirect recurrent impulse flow.

5-HT, DOPAMINE, AND GABAIn the absense of external stimulus,the easiest way to modulaterecurrent impulse flow is viatargeted breathing and/or postureexcercises found in yoga and othermeditative practices. One of thereasons there is such a strongcrossover between psychedelicsand Buddhism is because bothchallenge you to master the mostbasic impulses of your own mind.

BIOFEEDBACK MEDIATED FLOWWhen trying to understand howpsychedelics act as “non-specificamplifyers” of the psyche, it can beassumed that recurrent excitationcaused by hallucinogens followsthe path of bloodflow through thecranium. As an analytical circuitbecomes excited and demandsmore blood, more drug is borne tothat circuit via the bloodstream,further exciting the targeted circuit.

CRANIAL BLOOD FLOWThe most effective drivers ofsynchrony in recurrent impulse floware rythmically modulated sensoryinputs, such as drumming, chanting,droning, and strobing lights.Sonically induced neural spikestend to generate the greatest levelsof synchrony between all sensorycircuits, making music and dancingthe perfect media for fine-tuningrecurrent impulse synchrony, bothindividually and within groups.

RYTHMICALLY MEDIATED FLOW

Input fromOptic Nerve

LateralGeniculate

NucleusLGN

VisualCortex

OccipitalLobe

WorkingMemory

PFC

RecognitionCortex

TemporalLobe

SpatialCortexParietal

Lobe

The above circuit illustration shows the various layers of recurrent analysis along the visualprocessing pathway. The areas in orange and red indicate where recurrent excitation would behighest, based on the distribution density of 5-HT2A receptors in the cortex. The cascade ofrecurrent signal feedback along the entire visual pathway can lead to extreme perceptual distortionand eventual loss of network stability. While this schematic is for the visual pathway, a similarschematic could be applied to audio, somatic, and memory pathways as well.

THE ROLE OF 5-HT IN MODULATING RECURRENT EXCITATION

The most potent visual hallucinogens are the tryptamines,such as DMT, psilocybin, and LSD, which closely resembleserotonin. The indole ring each tryptamine carries is like atiny key for a specific receptor subtype known as 5-HT2A.5-HT (5-hydroxy-tryptamine) is the chemical name forserotonin, and the 5-HT receptor subtype targeted byhallucinogens is densest in the recipricol dendrite arborsof the sensory processing cortices.

5-HT2A partial agonists have been shown to cause a slowasynchronous leakage of glutamate from pre-synapticterminals in layer V pyramid cells. Glutamate is an excitatorytransmitter, and this small asynchronous leakage amplifiesthe duration and intensity of incoming sensory stimulus.When this slight amplification is applied to recurrent circuitsin a neural network, the excitatory effect is multiplied andrapidly spreads across the entire network.

Layer V pyramid neurons are not only dense with 5-HT2A

receptor sites, they are also critical in handling feedbackexcitation both within the cortex and between cortical andsub-cortical regions. When viewing cortical circuits froma schematic standpoint, it is easy to see how even mildexcitation in layer V signal processing can quickly spreadthroughout the entire cortical circuit to increase recurrentexcitat ion and ampli fy sensory signal output.

Glutamate

5-HT (Serotonin)

Hallucinogen

5-HT2A receptorIndole Ring

Tryptamine

Serotonin (5-HT)

DMTPsilocin

PsilocybinLSD SynchronousFast Current

AsynchronousSlow Current

Tonic Excitation

Synapse Close-UpLayer V pyramid cell

Tryptamine Structure

Reuptake

Impulse

LeakageRelease

Active at both pre- and post-synapticsites, the hallucinogenic aminecauses asynchrous glutamateleakage in presynaptic terminals aswell as slow tonic excitation at thepost-synaptic membrane. The resultis a slow amplification of excitatorysignal flow propagating throughoutthe entire network.

Layer V pyramid neurons handle cortical feedbackwith the thalamus and sub-cortical regions, as wellas intra-cortical feedback within the cortical layers.Cortical feedback excitation from layer V neurons ismediated through recurrent collaterals, axons whichreverse course and ascend back up into the highercortical layers. These recurrent projections make upthe vast majority of cortical axons, demonstrating theextreme importance of robust signal feedback incortical processing.

Layer V Recurrent Collaterals

I

II

III

IV

VI

VV

APICALDENDRITES

RECURRENTCOLLATERALS

THALAMUSINPUT

EXCITEDFEEDBACK

EXCITEDOUTPUT

EXCITEDFEEDBACK

SIMPLIFIED CORTICAL SCHEMATIC

THALAMOCORTICAL FEEDBACK

EXCITEDOUTPUT

EXCITEDFEEDBACK

5-HT AND ASYNCHRONOUS EXCITATION LAYER V NEURONS AND RECURRENT EXCITATIONSEROTONIN AND TRYPTAMINE HALLUCINOGENS

produced by James Kent · Trip Magazine · http://tripzine.com

Contents of this poster taken from Psychedelic Information Theory: Shamanism in the Age of Reason, by James Kent. http://tripzine.com/pit. Contact: jamesk@tripzine.com